This patch came from https://git.ghostscript.com/?p=mupdf.git;a=patch;h=cee7cefc610d42fd383b3c80c12cbc675443176a
and fixes CVE-2021-3407.

From cee7cefc610d42fd383b3c80c12cbc675443176a Mon Sep 17 00:00:00 2001
From: Robin Watts <Robin.Watts@artifex.com>
Date: Fri, 22 Jan 2021 17:05:15 +0000
Subject: [PATCH] Bug 703366: Fix double free of object during linearization.

This appears to happen because we parse an illegal object from
a broken file and assign it to object 0, which is defined to
be free.

Here, we fix the parsing code so this can't happen.
---
 source/pdf/pdf-parse.c | 6 ++++++
 source/pdf/pdf-xref.c  | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/source/pdf/pdf-parse.c b/source/pdf/pdf-parse.c
index 7abc8c3d4..5761c3351 100644
--- a/source/pdf/pdf-parse.c
+++ b/source/pdf/pdf-parse.c
@@ -749,6 +749,12 @@ pdf_parse_ind_obj(fz_context *ctx, pdf_document *doc,
 		fz_throw(ctx, FZ_ERROR_SYNTAX, "expected generation number (%d ? obj)", num);
 	}
 	gen = buf->i;
+	if (gen < 0 || gen >= 65536)
+	{
+		if (try_repair)
+			*try_repair = 1;
+		fz_throw(ctx, FZ_ERROR_SYNTAX, "invalid generation number (%d)", gen);
+	}
 
 	tok = pdf_lex(ctx, file, buf);
 	if (tok != PDF_TOK_OBJ)
diff --git a/source/pdf/pdf-xref.c b/source/pdf/pdf-xref.c
index 1b2bdcd59..30197b4b8 100644
--- a/source/pdf/pdf-xref.c
+++ b/source/pdf/pdf-xref.c
@@ -1190,6 +1190,8 @@ pdf_read_new_xref(fz_context *ctx, pdf_document *doc, pdf_lexbuf *buf)
 	{
 		ofs = fz_tell(ctx, doc->file);
 		trailer = pdf_parse_ind_obj(ctx, doc, doc->file, buf, &num, &gen, &stm_ofs, NULL);
+		if (num == 0)
+			fz_throw(ctx, FZ_ERROR_GENERIC, "Trailer object number cannot be 0\n");
 	}
 	fz_catch(ctx)
 	{
-- 
2.17.1

